Waste materials, particularly solid wastes, from various sources are continuing to increase at the same time that disposal of such material is becoming more difficult and expensive. As a result, there is increasing interest in recycling useful components of wastes and in using certain fractions for production of energy or higher-value materials such as commodity chemicals. The growing interest in segregation of solid waste, frequently at the source, will potentially provide relatively well-defined materials that are prime candidates for other uses. Of particular interest is the large amount of cellulosic materials, already segregated, that could be considered as low-cost, perhaps even negative cost, feed materials for the production of sugars and various other useful chemicals such as alcohols, neutral solvents and organic acids. Cellulosic materials are defined as those materials which contain cellulose. Cellulosic materials include wood, woody pulp, woody biomass, paper, cardboard, and related materials. As a result of recycling, the problem of disposal of solid wastes would be partially alleviated.
Solid waste material from residential and industrial sources represent a heterogenous mixture that is predominantly made up of metals, glass, plastics, food residues, and paper products. Although conservation efforts have had a significant impact, the volume of this material remains quite large and will probably continue to increase in the foreseeable future. A large amount of this material is either deposited in landfills or incinerated, whereas only a small amount is recycled or further used. Due to environmental restrictions and a lack of suitable new sites, disposal by landfill or incineration is becoming prohibitively expensive or even impossible in certain areas.
It has been estimated that half of municipal solid waste is made up of paper, with the other half consisting of glass, plastics, metals and other materials. A significant portion of the waste paper is comprised of newsprint. Large fractions of various types of the solid waste materials could be effectively recycled if fractionation and segregation of the components was carried out. Although there is some technology available that will fractionate mixed waste, there appears to be a trend towards segregation by the generator. If this occurs on a large scale, materials that are not readily recycled could well be considered as relatively well-defined feed materials for further processing.
Segregated waste paper products could be an ideal feed material for biological conversion to sugars (conversion of cellulose and hemicellulose) or aromatic compounds (conversion of lignin) with the possibility of subsequent conversion to a variety of useful chemicals. Of particular interest would be the production of organic acids, neutral solvents and various alcohols as chemical intermediates.
Waste paper is made up of three primary constituents: cellulose (.about.61%), hemicellulose (.about.16%), and lignin (.about.21%). The first two, cellulose and hemicellulose, are complex carbohydrates that can be hydrolyzed to the monomer sugars, glucose and xylose by use of the appropriate enzyme systems. The primary sugar is glucose which represents an intermediate product that can also be converted to chemicals such as ethanol by a fermentation process. The process chemistry of interest is listed below: ##STR1##
Unless extensive purification is carried out, the usual cellulase enzymes (a crude extract from specific microorganisms) include a mixture that has several functions including those biocatalysts that interact with the end groups of the cellulose polymer, those biocatalysts that interact with the interior part of the cellulose molecule, and those that convert cellulose to glucose. Cellobiose, an intermediate disaccharide, that is also formed (Equation 2) and glucose, both inhibit the hydrolysis reaction. Cellobiose can be converted to glucose if a sufficient quantity of the enzyme cellobiase is present (Equation 3). Cellobiase is also a constituent of the crude mixture of the cellulase enzymes but it is usually present at a relatively low concentration. In order to enhance the overall hydrolysis process, exposure to additional cellobiase would be highly beneficial. Lignin is a polymeric structure of aromatic compounds which can be oxidized to a series of useful chemical compounds, but this technology is not well-developed as yet, so that residue could be used as a fuel for producing steam.
Research on saccharification processes for the conversion of cellulose to glucose have taken two major approaches. Acid hydrolysis is attractive since it is relatively rapid. However, the acid processes also produce chemicals other than sugar that represent a process loss or complication. Treating the acid effluent or recovery of the excess acid also presents problems. On the other hand, the enzymatic approach is much more specific with a higher yield but, until recently, there has been concern over the length of time for the reaction and the potential high cost of the biocatalyst since there was no processing scheme for recovery and reuse. Both of the shortcomings of the enzyme process now appear to be solved so it is the obvious choice for new process development. The bioprocessing system of the present application is centered around the enzymatic hydrolysis of a major fraction of the cellulose in paper by the use of cellulase to produce sugar. Various intermediate processing steps involving ultrafiltration and reverse osmosis will be utilized to increase the overall reaction efficiency. Finally, subsequent fermentation will be carried out on the resulting sugar to chemicals, with a preliminary emphasis on useful chemicals such as ethanol.